1. Technical Field of the Invention
The present invention relates to developing devices for use in such image forming apparatus as various kinds of copying machines and laser printers for forming monocolor images or colored images by selectively using desired colors, and more particularly to a developing device provided with a plurality of developing units accommodating different colored toners.
2. Brief Description of Related Art
Generally, a color copying machine for forming colored image is designed to form a colored image by using four colored developers, and developing units accommodating each colored developer are disposed in a developing device in one of the following three methods.
(1) Stationary Method: Each developing unit is disposed at a predetermined position around a photoconductor for selective use.
(2) Rotary Method: Each developing unit is disposed on a rotary support member and a desired developing unit is positioned at a developing location opposite to a photoconductor by rotating the rotary support member.
(3) Reciprocative Movement Method: Each developing unit is disposed on a reciprocative movement support member, and a desired developing unit is positioned at a developing location opposite to a photoconductor by reciprocatively moving the reciprocative movement support member.
The stationary method has problems that the diameter of the photoconductor becomes large and that there occurs attenuation variation of electrostatic latent images with the passage of time because of the difference in distance between the position where each electrostatic latent image is formed and the developing location of each color, and it has to be rectified.
The rotary method is able to solve the above-mentioned problems inherent in the stationary method, however, there is a problem that toner supply can not be performed efficiently since developer tends to easily fall out of a developing unit when a rotary support member is rotated with developing units mounted thereon and toner is supplied only to a developing unit which is positioned at a predetermined rotative position.
The reciprocative movement method has the disadvantageous point that it has to provide a little larger space in the direction of reciprocating movement compared with the rotary method. However, it is provided with characteristics which can solve the problems inherent in the rotary method. Especially it is arranged for stirring and supplying toners to each developing unit even when developing operation is not being performed. It is, therefore, very advantageous for use in forming colored images where more toners are consumed than the case when monocolor images are formed.
Incidentally, in the developing device of the reciprocative movement method, the developing units which are not performing developing process are separated from the photoconductor unlike the stationary method. It is, therefore, principally not necessary for the developing unit to lower the height of developer accommodated therein at the location of development.
However, it may happen to compact and harden the developer when the developing unit is moved from an undeveloping position to the developing location again. Once the developer is hardened, it causes to clog a height regulating plate resulting in the void on an image. Such blocking phenomenon is apt to occur easily since the developer toner is finely prepared for more improved accuracy for developing process.
Even the developing units at the position outside the developing location, when the developer accommodated therein has its height sufficient for the development at the position facing outside of a developing roll, developer may happen to fall out of the developing unit when it is moved and stopped or by shock and vibration caused by some other reason. The developer fallen out of the developing unit sticks to the leading end portion of the unit opposite to a photoconductor and stains the inside of a copying machine. The developer adhered to the front end portion of the developing unit is attracted by the electric potential on the side of the photoconductor apart from the supply of developer from a developing roll when the developing unit is moved to the location of development and faced the photoconductor, and sticks to the photoconductor thereby staining an image.
Japanese Published Patent Applications TOKKAI SHO 61-208063 and TOKKAI SHO 55-115063 disclose developing devices capable of solving the problems. The former is designed to lower the height of developer on a developing roll at the portion adjacent to the photoconductor especially when the developing unit is positioned at the developing location opposite to the photoconductor by changing the positions of magnets arranged in the developing roll at the times of development and undevelopment, while the latter is provided with a shutter blade which is actuated for scraping off the developer on the developing roll when the developing unit is not positioned at the developing location.
These developing devices are arranged to switch the positions of magenets and shutter blade corresponding to the position of the developing unit by controlling a driving mechanism specially provided. However, it invites a rise in manufacturing cost since the special driving and control mechanisms have to be provided on each one of the developing units. Especially for the color image forming apparatus, considerable rise in manufacturing cost is inevitable since it is provided with many developing units.